Urea water supply piping is used for an exhaust purification system adapted to purify nitrogen oxides (hereinafter referred to as “NOx”) in internal combustion engines such as diesel engines. Such an exhaust purification system is disclosed, for example, in Patent Literature 1, where the disclosed exhaust purification system includes a catalyst provided in an exhaust passage of an internal combustion engine and a urea water addition valve provided on an upstream side of the catalyst. Urea water is stored in a tank, force-fed to a urea water addition valve by a pump through a urea water supply pipe, and added into the exhaust passage through the urea water addition valve. Consequently, the urea water is broken down into ammonia, and NOx in exhaust is selectively reduced on the catalyst by the ammonia, thereby purifying the exhaust.
Urea water freezes at around −11° C., presenting a problem in that the urea water can freeze in urea water supply piping depending on the use environment, which may make it impossible to supply the urea water to the exhaust passage.
To deal with this, a joint connected to a urea water supply pipe and provided with a heating wire for use to heat urea water is disclosed (e.g. Patent Literature 2). Also, the joint is provided with a joint cover for protection against the wind. The joint cover is made up of two housings shaped to be symmetrical to each other and the housings are mounted on the joint by attaching the joint to a first of the housings and then fixing a second of the housings to the first of the housings in such a way as to cover the joint from above.